While a general microscope is a device that measures the shape of an object by measuring the intensity of light reflected or transmitted from an object by irradiating a general light source to the object, a digital holography microscope is a device that measures interference and diffraction phenomenons of light when light is irradiated to an object, records the results thereof digitally, and reconstructs shape information of the object from the information thereof.
That is, digital holography technology generates single-wavelength light such as laser, splits the same into two beams by using a light splitter, directly irradiates one beam to an image sensor (referred to a reference beam), irradiates the other beam to a measurement target object and irradiates the beam reflected from the measurement target object to the image sensor (referred to as an object beam), records beam interference pattern information, which is generated due to an interference phenomenon between the reference beam and the object beam in the image sensor, through a digital image sensor, and reconstructs the shape of the measurement target object from the recorded interference pattern information by using a computer. In this case, the recorded interference pattern information is generally referred to as a hologram.
The digital holography microscope is different from an optical holography method of the related art in that it measures beam interference pattern information through a digital image sensor, stores the same digitally, and reconstructs the shape of a measurement target object by processing the stored interference pattern information not through an optical method but through a numerical operation method using a computer device.
Examples of the digital holography technologies of the related art include Korean Patent No. 10-0838586 registered on Jun. 10, 2008 and entitled “3D Measurement Device Using Digital Holography and 3D Measurement Method” (hereinafter referred to as “Related Art 1”), Korean Patent No. 10-0867302 registered on Oct. 31, 2008 and entitled “3D Measurement Device Using Digital Holography” (hereinafter referred to as “Related Art 2”), Korean Patent No. 10-0906508 registered on Jun. 30, 2009 and entitled “3D Measurement Device Using Digital Holography” (hereinafter referred to as “Related Art 3”), Korean Patent No. 10-1003241 registered on Nov. 21, 2008 and entitled “3D Measurement Device Using On-axis Digital Holography” (hereinafter referred to as “Related Art 4”), and Korean Patent No. 10-0870967 registered on Nov. 21, 2008 and entitled “Digital Hologram Recording/Reproducing Device and Recording/Reproducing Method for Removing Virtual Image” (hereinafter referred to as “Related Art 5”).
For example, FIG. 1 is a diagram illustrating a configuration of a 3D measurement device using digital holography, according to Related Art 1.
Referring to FIG. 1, the 3D measurement device using digital holography according to Related Art 1 includes a light source 10, a first imaging unit 23, a second imaging unit 24, a light splitting unit 13, a first wave plate 18, and a second wave plate 20.
A beam emitted from the light source 10 becomes a parallel beam by passing through a pinhole (not illustrated) of a fixing member 11 and a convex lens 12 arranged between the fixing member 11 and the light splitting part 13, and the parallel beam is incident on the light splitting unit 13. The light splitting unit 13 (e.g., a polarizer beam splitter (PBS)) splits the beam emitted from the light source 10 into a reference beam and an object beam the polarization directions of which are perpendicular to each other, and directs the reference beam and the object beam to a reference plane 3 and a measurement target object 1, respectively.
A third wave plate 15 is arranged between the light splitting unit 13 and the reference plane 3, and a fourth wave plate 14 is arranged between the light splitting unit 13 and the measurement target object 1. The third wave plate 15 and the fourth wave plate 14 may be implemented as ¼-wave plates. The reference beam directed from the light splitting unit 13 to the reference plane 3 passes through the third wave plate 15, and the reference beam passed through the third wave plate 15 is directed to the reference plane 3. The reference beam reflected from the reference plane 3 again passes through the third wave plate 15. In this case, the polarization direction between the reference beam incident on the third wave plate 15 from the light splitting unit 13 and the reference beam reflected from the reference plane 3 and passed through the third wave plate 15 has a difference of 90.
Also, the object beam directed from the light splitting unit 13 to the measurement target object 1 passes through the fourth wave plate 14, and the object light passed through the fourth wave plate 14 is directed to the measurement target object 1. The object beam reflected from the measurement target object 1 again passes through the fourth wave plate 14. In this case, as in the reference beam, the polarization direction between the object beam incident on the fourth wave plate 14 from the light splitting unit 13 and the object beam reflected from the measurement target object 1 and passed through the fourth wave plate 14 has a difference of 90.
As described above, since the third wave plate 15 and the fourth wave plate 14 are located between the light splitting unit 13 and the reference plane 3 and the measurement target object 1 respectively, the energy loss of light passing through the light splitting unit 13 in the form of a polarizer beam splitter may be minimized.
Meanwhile, the reference beam and the object beam respectively reflected from the reference plane 3 and the measurement target object 1 and passed through the third wave plate 15 and the fourth wave plate 14 again pass through the light splitting unit 13 and are directed to the first wave plate 18. A mutual interference occurs as the reference beam and the object beam from the light splitting unit 13 pass through the first wave plate 18.
Also, an interference beam passes through a prism unit 19. As the interference beam passes through the prism unit 19, it is split into a first interference beam directed to the first imaging unit 23 and a second interference beam directed to the second imaging unit 24. In this case, the first interference beam and the second interference beam have a phase difference of 180. The first interference beam directed to the first imaging unit 23 is captured by the first imaging unit 23 and acquired as image information having one phase, for example, a phase of 0. On the other hand, the second interference light directed to the second imaging unit 24 passes through the second wave plate 20 with a phase delay of 90 and is incident on the second imaging unit 24. As a result, the phase difference between the second interference beam captured by the second imaging unit 24 and the first interference beam captured by the first imaging unit 23 becomes 90 due to the second wave plate 20, and thus two pieces of image information for acquiring a 3D image may be simultaneously acquired by the first imaging unit 23 and the second imaging unit 24. Herein, each of the first wave plate 18 and the second wave plate 20 may be implemented as a ¼-wave plate.
The first imaging unit 23 and the second imaging unit 24 may be implemented as charge coupled device (CCD) cameras, the first and second interference beams acquired by the first and second imaging units 23 and 24 may be used by a controller such as a computer (not illustrated) to acquire a 3D image of the measurement target object 1, and a Fresnel transform used in the digital holography method of the related art may be applied to acquire a 3D image based on digital holography.
In Related Art 1 described above, since image information having a phase difference of 90 is simultaneously captured in one shot by the first imaging unit 23 and the second imaging unit 24, the measurement speed limitation caused by the on-axis method of the related art requiring a plurality of holograms may be solved and the high-speed measurement capability of the off-axis method may be achieved. Also, since the reference beam and the object beam has a certain angle due to the off-axis method of the related art, the limitation of failing to use the entire region of the CCD camera, that is, degradation of lateral resolution, may be prevented.
The following limitations may arise in the generation and reconstruction of a hologram in holography methods of the related art (including an optical holography method and a digital holography method).
First, in the optical holography method of the related art, a time delay error may occur in the case of acquiring two holograms (object hologram and reference hologram) required for hologram reconstruction.
Also, in the off-axis holography method among the holography methods of the related art, due to the DC noise (including background noise) that is not removed in hologram reconstruction, acquisition of quantitative 3D information of an object may be difficult or an error may occur.
Also, in the on-axis holography method among the holography methods of the related art, since DC information and virtual image information are not separated in the acquired hologram, complete removal of the DC information and the virtual image information may be impossible, use of two image sensors (in the case of Related Arts 1 to 3) or three image sensors (in the case of Related Art 4) may be required, and use of one, two, or four physical optical devices is required to use a phase shifting method (particularly, use of four ¼-wave plates may be required in the case of Related Art 1, use of one ¼-wave plate may be required in the case of Related Art 2, use of two ¼-wave plates may be required in the case of Related Art 3, and use of one ¼-wave plate and one ½-wave plate (i.e., two wave plates) may be required in the case of Related Art 4. Thus, in the related art, the overall structure of a hologram generating and reconstructing device may be complex and the total manufacturing cost may be high.
Also, in the case of Related Art 5, although it may be possible to reconstruct a 3D hologram image where virtual image information is removed by using a 4-split method, since the object information included in the DC information may be lost in the process of removing the DC information and the DC information may not be completely removed due to the technical limitations, accurate 3D information of a measurement object may not be acquired.
Thus, a new method for solving the above limitations may be required.